Dissolved organic carbon and nitrogen dynamics in the Douro River estuary, Portugal

Abstract

Dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) are metabolically important in estuaries. Their availability is influenced by organic matter inputs (internal and external) as well as by internal microbial transformations within the system. In this study, spatial and seasonal dynamics of DOC and DON were evaluated in the Douro River estuary (Portugal). Monthly surveys were performed from October 2005 to December 2006 along a salinity gradient at three different depths. DOC and DON were calculated by subtracting the respective inorganic components (dissolved inorganic carbon, DIC, and dissolved inorganic nitrogen, DIN) from total dissolved carbon (TDC) and total dissolved nitrogen (TDN). In the Douro River estuary, DIC exhibits a linear and positive distribution as a function of salinity, indicating an input of DIC into the estuary from coastal waters. This pattern of distribution along the salinity gradient was also observed for total particulate matter, since a decreasing trend upstream was observed. In contrast, the results showed a general nonconservative behaviour of DOC along the estuarine transect during most months. This typical deviation from conservative mixing was mainly associated with sewage DOC sources into the lower and middle estuary. On average, DON concentrations represented 52-88% of the TDN. Along the transect, relationships between DIN and DON with salinity were generally conservative, decreasing towards the mouth, highlighting the fluvial source of such nitrogen forms; however, occasional DON sewage loads were registered. Estimated global Douro River watershed exports of DOC and DON to coastal waters yielded values of 420 kg C m-2 yr-1 and 125 kg N m-2 yr-1 , respectively, which are high when compared with previously predicted levels from global models of DOC and DON export.

Variability of plankton production during the spring bloom in NW Iberia

“…We reviewed NO 3 values for the Galician rivers in the yearly reports on the oceanographic conditions for Galicia from INTECMAR monitoring (www.intecmar.gal/). For the Douro in Portugal, our value is higher than that reported by Magalhaes et al (2008), who measured a maximum of 0.5 mg l −1 in April (8 mmol m −3 ), and slightly lower than the values in the data set from the Portuguese National Water Resources Information System (https://snirh.apambiente.pt/), where values of around 15.5 mmol m −3 were obtained by averaging the measurements of several stations located at the lower basin of the estuary. Nutrient load for the rest of the state variables was considered to be negligible.…”

Response of anaerobic ammonium oxidation to inorganic nitrogen fluctuations in temperate estuarine sediments

“…The highest water column NO 3 − concentrations were systematically measured in the stations with the lowest salinities, showing the importance of riverine inputs. The low salinity value and high NO 3 − concentration observed at the downstream site in the Douro estuary are the result of high flow discharges by upstream dams in the sampling period transforming the estuary in a single water mass [ Magalhães et al ., ]. The heavily modified Ave estuary was characterized by higher levels of DOC and NO 3 − .…”

Larval fish dispersal along an estuarine–ocean gradient

“…For TPM and POM assessment, samples were previously filtered through precombusted GF/F glass-fibre filters, which were dried at 105ºC (TPM) and then incinerated at 500ºC (POM), according to APHA (1992). TDC and DOC were determined using a Shimadzu Instruments TOC-VCSN analyzer following Magalhães et al (2008).…”